One of several known processes for the production of tinplated steel, high-speed halogen electroplating, typically uses plating baths which comprise stannous chloride, sodium bifluoride, sodium fluoride, sodium chloride and hydrochloric acid together with a grain-refining additive. In order to minimize corrosion of the steel as it is plated, however, the degree of acidity must be moderate (i.e., pH of between 3 to 4). Moderate acidity, in turn, requires that the stannous tin be combined with fluoride ions in a chemical complex in order to minimize the reaction of stannous tin with oxygen to form stannic tin which precipitates and is lost in the plating sludge. Dissolved iron in the plating bath accelerates the oxidation of fluoride-complexed stannous ions so that a substantial portion of the stannous ions are lost in the plating sludge. Thus, the iron, if not removed from the halogen plating bath, de-stabilizes the process resulting in off-quality tinplate, low productivity, and high costs for replenishment of tin and other chemicals.
To counteract the effect of iron, ferrocyanide is added in large quantities to the bath to remove dissolved iron from the electrolyte by forming insoluble compounds which report to the plating sludge. It is normally added as the sodium ferrocyanide decahydrate salt and results in the immediate and total precipitation of iron ions from the electrolyte before they react with oxygen and/or stannous tin. The use of alkali ferricyanides or ferrocyanides for this purpose is disclosed in U.S. Pat. Nos. 2,402,185 and 2,512,719.
These ferrocyanide additions result, however, in precipitated iron ferrocyanides which, along with insoluble stannic tin in the form of sodium fluostannate, become the major ingredients of a heavy sludge that accumulates in the plating cells, storage tanks, and throughout the recirculating system. Typically, halogen electroplating lines must be shut down periodically so that this sludge can be removed. In recent years, concern has been growing about the environmental impact of the ferrocyanide content of the plating section sludge, slurries, and waste waters.
Thus, there is a need to develop other ways to remove iron and/or stabilize a halogen plating bath so that the use of ferrocyanide can be discontinued.